1. Field of the Invention
The present invention relates to a semiconductor device having a solid imaging element, i.e. to a solid imaging device, and further to a method of producing the same.
2. Description of the Background Art
FIG. 13 is a view illustrating a circuit construction of a solid imaging device. Unit pixels or unit cells C are arranged in a matrix form, and each unit cell C is connected to a vertical shift register and a horizontal shift register.
Each unit cell C has a photodiode PD, a transfer switch M1, a reset switch M2, an amplifier M3, and a selection switch M4. The transfer switch M1, the reset switch M2, the amplifier M3, and the selection switch M4 are each formed of a MOS transistor. The photodiode PD serves to perform a function of converting incident light into an electric signal. The signal charge Q obtained by the photodiode PD is transferred by the transfer switch M1 to a diffusion layer FD (Floating Diffusion) for signal charge conversion. Assuming the capacitance of the FD to be C, the signal charge is converted into a voltage defined by V=Q/C. FIG. 14 illustrates a pattern layout of each element. FIG. 15 illustrates a cross-sectional view taken along the line XVxe2x80x94XV of FIG. 14.
Referring to FIG. 15, an isolation oxide film 2 is formed in a principal surface of a semiconductor substrate 1 by the LOCOS (local oxidation of silicon) method. A predetermined impurity is implanted into each of the active regions of the semiconductor substrate 1 surrounded and exposed by the isolation oxide film 2. The active regions include at least a region serving as the PD (hereafter referred to as a xe2x80x9cPD regionxe2x80x9d) and a region serving as the FD (hereafter referred to as a xe2x80x9cFD regionxe2x80x9d). The PD region and the FD region are in an adjacent positional relationship to sandwich a gate electrode 8 formed above the principal surface of the semiconductor substrate 1. The side wall of the gate electrode 8 is covered with a side wall spacer.
The inside of the semiconductor substrate 1 contains a stress damage layer 31 generated by action of a mechanical load at the time of forming the isolation oxide film 2, an etching damage layer 32 generated by etching for forming the side wall spacer, and an ion implantation damage layer 33 generated by ion implantation into the active regions. When a part xcex94Q of the electric charge Q is lost as a leakage current through the defects of these damage layers, the sensitivity of the solid imaging device decreases to deteriorate the pixel characteristics.
Therefore, an object of the present invention is to provide a solid imaging device in which the damage layers are reduced as much as possible.
In order to achieve the aforesaid object, a solid imaging device according to one aspect of the present invention includes a semiconductor substrate having a principal surface, a photodiode including a first diffusion layer formed on the principal surface, and a MOS (Metal Oxide Semiconductor) transistor including a second diffusion layer and a third diffusion layer formed on the principal surface as source/drain regions. The second diffusion layer serves to perform a function of converting a signal charge, which is determined by the photodiode, into a signal voltage. The second diffusion layer has an impurity concentration less than or equal to {fraction (1/10)} of an impurity concentration of the third diffusion layer
By adopting the aforesaid construction, a high concentration diffusion layer is not formed in the FD region serving as the second diffusion layer, thereby restraining the formation of the ion implantation damage layer in the semiconductor substrate at the FD region. Therefore, the leakage current caused by the defects of the damage layers can be reduced. Also, since the concentration of the diffusion layer itself decreases, the electric field at the PN junction interface is alleviated to reduce the leakage current.
In the aforesaid invention, an upper side of the second diffusion layer is preferably covered with an implantation shielding layer for shielding against ion implantation. By adopting this construction, a side wall spacer is not formed on the FD region serving as the second diffusion layer, and a high concentration diffusion layer is not formed in the FD region, either. Therefore, the etching damage layer generated at the time of forming the side wall spacer and the ion implantation damage layer generated at the time of forming the high concentration diffusion layer are not generated in the FD region. Thus, the generation of damage layers can be restrained, thereby reducing the leakage current.
In the aforesaid invention, it is preferable that the MOS transistor includes a side wall spacer formed by etching, an etching protective layer intervenes between the second diffusion layer and the side wall spacer and between the third diffusion layer and the side wall spacer for preventing generation of damages to the principal surface by the etching, and an upper side of the second diffusion layer and an upper side of the third diffusion layer are covered with the etching protective layer. By adopting this construction, generation of an etching damage layer in the FD region serving as the second diffusion layer can be avoided. Since the etching damage layer as well as the ion implantation damage layer generated at the time of forming the high concentration diffusion layer can be reduced, the total amount of damage layers can be reduced, thereby leading to decrease in the leakage current.
In order to achieve the aforesaid object of the present invention, a solid imaging device according to another aspect of the present invention includes a semiconductor substrate having a principal surface, a photodiode including a first diffusion layer formed on the principal surface, and a MOS transistor including a second diffusion layer and a third diffusion layer formed on the principal surface as source/drain regions. The MOS transistor includes a side wall spacer formed by etching. An etching protective layer intervenes between the second diffusion layer and the side wall spacer and between the third diffusion layer and the side wall spacer for preventing generation of damages to the principal surface by the etching. An upper side of the second diffusion layer and an upper side of the third diffusion layer are covered with the etching protective layer. By adopting this construction, generation of an etching damage layer in the FD region serving as the second diffusion layer can be avoided. As a result of this, the total amount of damage layers can be reduced, thereby leading to decrease in the leakage current.
In order to achieve the aforesaid object, a solid imaging device according to still another aspect of the present invention includes a semiconductor substrate having a principal surface, a photodiode including a first diffusion layer formed on the principal surface, and a MOS transistor including a second diffusion layer and a third diffusion layer formed on the principal surface as source/drain regions. The second diffusion layer serves to perform a function of converting a signal charge, which is determined by the photodiode, into a signal voltage. The MOS transistor includes an isolation oxide film formed on the principal surface, a gate electrode formed on an upper side of the isolation oxide film, and a side wall spacer that covers a side part of the gate electrode. The second diffusion layer includes a first part in which a first impurity implantation has been carried out and a second part in which a second impurity implantation has been carried out. The second part extends in a range that includes and goes beyond the first part, as viewed from above.
By adopting this construction, the additional implantation diffusion layer serving as the second part is diffused to go beyond the range of the low concentration diffusion layer and the high concentration diffusion layer serving as the first part, as viewed from above. Therefore, the defects of the stress damage layer located near the isolation oxide film are covered with the additional implantation diffusion layer, so that no defects that may cause leakage in the electric current are present in a range to which the depletion layer of the PN junction between the semiconductor substrate and the high concentration diffusion layer extends. Therefore, the leakage current can be reduced.
In the aforesaid invention, the gate electrode and the side wall spacer are preferably formed to be limited within an area of the isolation oxide film, as viewed from above. By adopting this construction, there will be no part shadowed by the side wall spacer before the isolation oxide film, so that the additional implantation diffusion layer serving as the second part can easily reach the stress damage layer generated in the neighborhood of the isolation oxide film. Therefore, the additional implantation diffusion layer can cover the stress damage layer to a larger depth, and the defects of the stress damage layer can be made harmless with more certainty. As a result, the leakage current can be reduced, whereby the sensitivity as a solid imaging device is improved to provide better pixel characteristics.
In order to achieve the aforesaid object, a method of producing a solid imaging device according to one aspect of the present invention includes performing an isolation oxide film forming step of forming an isolation oxide film in a principal surface of a semiconductor substrate, performing a gate electrode forming step of forming a gate electrode at a position selected from an upper surface of the isolation oxide film, performing a first implantation step of implanting an impurity at a first concentration into a first region and a second region which are in the principal surface and defined by the isolation oxide film and which are in an opposite positional relationship with respect to each other to sandwich the gate electrode, thereafter performing a covering step of covering an entire surface of the semiconductor substrate with an oxide film, performing a side wall spacer forming step of partially removing the oxide film so as to expose the principal surface of the first region while leaving a part of the oxide film that covers the second region, and to form a side wall spacer that covers a side part of the gate electrode, and performing a second implantation step of implanting an impurity at a second concentration into the principal surface of the first region.
By adopting the aforesaid method, the impurity implantation at the second concentration is carried out in a state in which the second region is covered with the oxide film, so that the diffusion layer having a high concentration serving as the second concentration is not formed in the FD region serving as the second region. Therefore, the generation of damage layers accompanying the ion implantation can be restrained to reduce the leakage current.
In order to achieve the aforesaid object, a method of producing a solid imaging device according to another aspect of the present invention includes performing an isolation oxide film forming step of forming an isolation oxide film in a principal surface of a semiconductor substrate, performing a gate electrode forming step of forming a gate electrode at a position selected from an upper surface of the isolation oxide film, performing a first implantation step of implanting an impurity at a first concentration into a first region and a second region which are in the principal surface and defined by the isolation oxide film and which are in an opposite positional relationship with respect to each other to sandwich the gate electrode, thereafter performing an oxide film covering step of covering an entire surface of the semiconductor substrate with an oxide film, thereafter performing a nitride film covering step of forming a nitride film to cover the entire surface of the semiconductor substrate, performing a side wall spacer forming step of partially removing the nitride film so as to form a side wall spacer that covers a side part of the gate electrode from the nitride film and to leave the oxide film, and performing a second implantation step of implanting an impurity at a second concentration into the principal surface of the first region.
By adopting the aforesaid method, the etching for forming the side wall spacer is carried out so that the oxide film serving as the etching protective layer remains under the side wall spacer, so that the effects of the etching are prevented from reaching the semiconductor substrate side by the oxide film. Therefore, the generation of the etching damage layer in the FD region serving as the second region can be avoided. This can reduce the damage layers, leading to decrease in the leakage current.
In the aforesaid invention, the second implantation step preferably includes masking the second region to prevent the second concentration impurity from being implanted into the principal surface of the second region. By adopting this method, the diffusion layer having a high concentration serving as the second concentration is not formed in the FD region serving as the second region. Therefore, the generation of ion implantation damages in the FD region can be prevented as well. This leads to more reduction in the leakage current, so that it is preferable.
In order to achieve the aforesaid object, a method of producing a solid imaging device according to still another aspect of the present invention includes performing an isolation oxide film forming step of forming an isolation oxide film in a principal surface of a semiconductor substrate, performing a gate electrode forming step of forming a gate electrode at a position selected from an upper surface of the isolation oxide film, performing a side wall spacer forming step of forming a side wall spacer to cover a side part of the gate electrode, performing a first implantation step of implanting a first impurity under a first condition into an active region which is in the principal surface and defined by the isolation oxide film and which is adjacent to the gate electrode, and performing a second implantation step of implanting a second impurity into the active region under a second condition such that the second impurity is diffused in a range that includes and goes beyond a part to which the first impurity implanted in the first implantation step is diffused, as viewed from above.
By adopting the aforesaid method, the additional implantation diffusion layer implanted in the second implantation step is diffused to go beyond the range of the low concentration diffusion layer and the high concentration diffusion layer serving as the first part, as viewed from above. Therefore, the defects of the stress damage layer located near the isolation oxide film are covered with the additional implantation diffusion layer, so that no defects that may cause leakage in the electric current are present in a range to which the depletion layer of the PN junction between the semiconductor substrate and the high concentration diffusion layer extends. Therefore, the leakage current can be reduced.
In the aforesaid invention, the gate electrode and the side wall spacer are preferably formed to be limited within an area of the isolation oxide film, as viewed from above, in the gate electrode forming step and in the side wall spacer forming step. By adopting this method, there will be no part shadowed by the side wall spacer before the isolation oxide film, so that the additional implantation diffusion layer serving as the second part can easily reach the stress damage layer generated in the neighborhood of the isolation oxide film. Therefore, the additional implantation diffusion layer can cover the stress damage layer to a larger depth, and the leakage current can be reduced.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.